1. Chapter at a Glance
Hydrocarbons contain only carbon and hydrogen. They are classified as alkanes (single bonds), alkenes (C=C), alkynes (C≡C) and aromatic hydrocarbons (benzene and its relatives). This chapter covers their preparation, physical properties, conformations and reactions — free-radical substitution in alkanes, electrophilic addition in alkenes and alkynes, and electrophilic substitution in benzene. JEE Main tests reaction products, Markovnikov's rule, ozonolysis and aromaticity.
- Markovnikov and anti-Markovnikov (peroxide effect) addition of HBr.
- Products of ozonolysis; identifying the alkene from its products.
- Acidity of terminal alkynes; conformations of ethane.
- Aromaticity (Hückel rule) and ortho/para vs meta directing groups.
2. Alkanes (CnH2n+2)
Preparation
- Hydrogenation of alkenes/alkynes with Ni, Pt or Pd.
- Wurtz reaction: 2RX + 2Na → R–R + 2NaX (dry ether). Gives even-carbon alkanes; a mixture of two halides gives three alkanes.
- Decarboxylation: RCOONa + NaOH (CaO) → RH + Na₂CO₃ (one carbon less).
- Kolbe's electrolysis: 2RCOONa → R–R (at the anode) + 2CO₂. Sodium acetate gives ethane.
Properties and reactions
- Boiling point rises with molecular mass and falls with branching (less surface contact): n-pentane > isopentane > neopentane.
- Free radical halogenation (light or heat): initiation, propagation, termination. Reactivity F₂ > Cl₂ > Br₂ > I₂; selectivity of H: 3° > 2° > 1°.
- Combustion, controlled oxidation, isomerisation (with AlCl₃/HCl) and aromatisation (n-hexane → benzene over Cr₂O₃/V₂O₅).
Conformations of ethane
Rotation about the C–C single bond gives staggered (most stable, H atoms far apart) and eclipsed (least stable) forms. The energy difference is about 12.5 kJ mol−1 (torsional strain), small enough that rotation is free at room temperature.
3. Alkenes (CnH2n)
Preparation
- Partial hydrogenation of alkynes: Lindlar's catalyst (Pd/BaSO₄ + quinoline) gives cis-alkene; Na in liquid NH₃ (Birch) gives trans-alkene.
- Dehydrohalogenation of alkyl halides with alcoholic KOH (Saytzeff product — more substituted alkene — is major).
- Dehydration of alcohols with conc. H₂SO₄ (via carbocation; 3° > 2° > 1°).
- Dehalogenation of vicinal dihalides with Zn.
Reactions (electrophilic addition)
- Markovnikov's rule: in adding HX to an unsymmetrical alkene, H goes to the carbon that already has more H atoms (via the more stable carbocation). Propene + HBr → 2-bromopropane.
- Peroxide (Kharasch) effect: with HBr in the presence of peroxide, addition is anti-Markovnikov (free-radical path): propene → 1-bromopropane. Not observed with HCl or HI.
- Addition of Br₂ in CCl₄ (anti addition, decolourises red-brown bromine — test for unsaturation).
- Ozonolysis: O₃ then Zn/H₂O cleaves C=C into two carbonyl compounds.
- Cold dilute alkaline KMnO₄ (Baeyer's reagent) gives a vicinal diol (syn); pink colour disappears.
- Hydroboration–oxidation (B₂H₆, then H₂O₂/OH⁻) gives the anti-Markovnikov alcohol.
- Polymerisation: ethene → polythene.
4. Alkynes (CnH2n−2)
- Preparation: CaC₂ + 2H₂O → HC≡CH + Ca(OH)₂; double dehydrohalogenation of vicinal dihalides.
- Acidity: the sp C–H of a terminal alkyne is acidic (50% s-character). HC≡CH > CH₂=CH₂ > CH₃CH₃. Terminal alkynes react with Na or NaNH₂ and give white precipitate with ammoniacal AgNO₃ (Tollens') and red with ammoniacal Cu₂Cl₂; internal alkynes do not.
- Hydration with dil. H₂SO₄/HgSO₄: ethyne → ethanal; other alkynes → ketones (via enol).
- Cyclic polymerisation: 3HC≡CH → benzene (red-hot iron tube, 873 K).
5. Aromatic Hydrocarbons
Hückel's rule: a compound is aromatic if it is cyclic, planar, fully conjugated and has (4n + 2) π electrons (2, 6, 10 …). Benzene (6), cyclopentadienyl anion (6), tropylium cation (6) and pyridine are aromatic. Cyclobutadiene (4) and cyclooctatetraene (non-planar) are not.
- Benzene undergoes electrophilic substitution (not addition) to keep its aromatic stability: nitration (conc. HNO₃ + conc. H₂SO₄, electrophile NO₂⁺), halogenation (X₂/FeX₃), sulphonation (SO₃), Friedel–Crafts alkylation (RCl/AlCl₃) and acylation (RCOCl/AlCl₃).
- Addition does happen under harsh conditions: benzene + 3H₂ (Ni) → cyclohexane; benzene + 3Cl₂ (UV) → BHC (gammexane).
| Substituent | Effect on ring | Directs to |
|---|---|---|
| –OH, –NH₂, –OR, –NHCOCH₃, –CH₃ | Activating | ortho, para |
| –F, –Cl, –Br, –I | Deactivating (−I beats +R) | ortho, para |
| –NO₂, –CN, –CHO, –COOH, –SO₃H, –N⁺R₃ | Deactivating | meta |
6. Reaction Master Table
| Reagent / reaction | Result |
|---|---|
| HBr (no peroxide) | Markovnikov product |
| HBr + peroxide | Anti-Markovnikov product |
| O₃, then Zn/H₂O | C=C → two C=O |
| Lindlar catalyst | alkyne → cis-alkene |
| Na / liquid NH₃ | alkyne → trans-alkene |
| HgSO₄ / dil. H₂SO₄ | alkyne → aldehyde or ketone |
| Wurtz (Na, dry ether) | 2RX → R–R |
| Hückel rule | (4n + 2) π electrons |
7. Mind Map
- Wurtz, Kolbe
- Free-radical halogenation
- Conformations
- Markovnikov, peroxide effect
- Ozonolysis
- Baeyer's test
- Acidic terminal H
- Hydration
- Lindlar vs Na/NH₃
- Hückel rule
- Electrophilic substitution
- Directing groups
8. Tips and Tricks
- Reverse ozonolysis: join the two carbonyl carbons with a double bond and remove the oxygens.
- Peroxide effect works only with HBr (not HCl, HI) — "Br is the Boss".
- Number of monochloro products = number of different kinds of H atoms.
- For aromaticity, count π electrons: each double bond gives 2, a negative charge on a ring carbon gives 2, a positive charge gives 0.
- Applying Markovnikov's rule to symmetrical alkenes (it does not matter there).
- Forgetting that Friedel–Crafts reactions fail on strongly deactivated rings such as nitrobenzene.
- Assuming halogens are meta directors because they deactivate the ring.
9. Important One-liners
- Staggered ethane is more stable than eclipsed ethane.
- Branching lowers the boiling point of isomeric alkanes.
- Ethyne is more acidic than ethene and ethane.
- Baeyer's reagent is cold, dilute, alkaline KMnO₄.
- The electrophile in nitration of benzene is NO₂⁺.
- Cyclopentadienyl anion is aromatic with 6 π electrons.
10. PYQ-pattern Questions with Solutions
These questions are original and modelled on concepts frequently tested in JEE Main.
Propene reacts with HBr in the presence of benzoyl peroxide to give mainly:
- 2-Bromopropane
- 1-Bromopropane
- 1,2-Dibromopropane
- Propane
Show solution
Peroxide causes free-radical addition. Br• adds to the terminal carbon to give the more stable 2° radical, so Br ends up on C1: anti-Markovnikov product 1-bromopropane.
Answer: (B)
Ozonolysis of 2-methylbut-2-ene followed by Zn/H₂O gives:
- Two molecules of acetone
- Acetone and acetaldehyde
- Two molecules of acetaldehyde
- Propanal and formaldehyde
Show solution
(CH₃)₂C=CH–CH₃: the double bond breaks into (CH₃)₂C=O (acetone) and O=CH–CH₃ (acetaldehyde).
Answer: (B)
Which gives a white precipitate with ammoniacal silver nitrate?
- But-2-yne
- But-1-yne
- But-1-ene
- Butane
Show solution
Only terminal alkynes have the acidic ≡C–H that forms silver acetylide (white precipitate). But-1-yne is terminal; but-2-yne is internal.
Answer: (B)
A mixture of CH₃Br and C₂H₅Br is heated with sodium in dry ether. How many different alkanes are formed?
Show solution
Possible couplings: CH₃–CH₃ (ethane), CH₃–C₂H₅ (propane) and C₂H₅–C₂H₅ (butane). Total 3.
Answer: 3
Which of the following is aromatic?
- Cyclobutadiene
- Cyclopentadienyl cation
- Cyclopentadienyl anion
- Cyclooctatetraene
Show solution
Cyclopentadienyl anion is cyclic, planar and conjugated with 6 π electrons (4n + 2, n = 1). Cyclobutadiene (4) and the cation (4) are antiaromatic; cyclooctatetraene (8) is tub-shaped and non-aromatic.
Answer: (C)
But-2-yne on treatment with Na in liquid NH₃ gives:
- cis-But-2-ene
- trans-But-2-ene
- Butane
- But-1-ene
Show solution
Dissolving-metal reduction (Birch-type) adds H atoms anti, giving the trans-alkene. Lindlar's catalyst would give the cis isomer.
Answer: (B)
How many structurally different monochloro products are formed on free-radical chlorination of 2-methylbutane?
Show solution
2-Methylbutane (CH₃)₂CH–CH₂–CH₃ has four kinds of H: the two equivalent CH₃ on C2, the C2–H, the CH₂ (C3) and the terminal CH₃ (C4). So 4 structural monochloro products.
Answer: 4
Nitration of toluene gives mainly:
- m-Nitrotoluene
- o- and p-Nitrotoluene
- Nitrobenzene
- Benzoic acid
Show solution
The –CH₃ group activates the ring (by +I and hyperconjugation) and stabilises the intermediate carbocation most when NO₂⁺ attacks ortho or para. So o- and p-nitrotoluene are the main products.
Answer: (B)
11. Quick Revision Summary
- Alkanes: Wurtz, Kolbe, decarboxylation; free-radical halogenation; staggered more stable.
- Alkenes: Markovnikov; HBr + peroxide → anti-Markovnikov; ozonolysis gives carbonyls.
- Alkynes: terminal H is acidic; Lindlar → cis, Na/NH₃ → trans; hydration → carbonyl.
- Aromatic: Hückel (4n + 2); electrophilic substitution.
- o/p directors: –OH, –NH₂, –CH₃, halogens; meta: –NO₂, –CHO, –COOH, –CN.